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  <h2>8-if-else 语句</h2>



  <p class="post-date">2019-03-10</p>
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    <section class="markdown-content"><p>if 是条件语句。if 语句的语法是</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> condition &#123;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>如果 <code>condition</code> 为真，则执行 <code>{</code> 和 <code>}</code> 之间的代码。</p>
<p>不同于其他语言，例如 C 语言，Go 语言里的 <code>{  }</code> 是必要的，即使在 <code>{  }</code> 之间只有一条语句。</p>
<p>if 语句还有可选的 <code>else if</code> 和 <code>else</code> 部分。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> condition &#123;  </span><br><span class="line">&#125; <span class="keyword">else</span> <span class="keyword">if</span> condition &#123;</span><br><span class="line">&#125; <span class="keyword">else</span> &#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>if-else 语句之间可以有任意数量的 <code>else if</code>。条件判断顺序是从上到下。如果 <code>if</code> 或 <code>else if</code> 条件判断的结果为真，则执行相应的代码块。 如果没有条件为真，则 <code>else</code> 代码块被执行。</p>
<p>让我们编写一个简单的程序来检测一个数字是奇数还是偶数。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (  </span><br><span class="line">    <span class="string">"fmt"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;  </span><br><span class="line">    num := <span class="number">10</span></span><br><span class="line">    <span class="keyword">if</span> num % <span class="number">2</span> == <span class="number">0</span> &#123; <span class="comment">//checks if number is even</span></span><br><span class="line">        fmt.Println(<span class="string">"the number is even"</span>) </span><br><span class="line">    &#125;  <span class="keyword">else</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"the number is odd"</span>)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><code>if num％2 == 0</code> 语句检测 num 取 2 的余数是否为零。 如果是为零则打印输出 “the number is even”，如果不为零则打印输出 “the number is odd”。在上面的这个程序中，打印输出的是 <code>the number is even</code>。</p>
<p><code>if</code> 还有另外一种形式，它包含一个 <code>statement</code> 可选语句部分，该组件在条件判断之前运行。它的语法是</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> statement; condition &#123;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>让我们重写程序，使用上面的语法来查找数字是偶数还是奇数。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (  </span><br><span class="line">    <span class="string">"fmt"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;  </span><br><span class="line">    <span class="keyword">if</span> num := <span class="number">10</span>; num % <span class="number">2</span> == <span class="number">0</span> &#123; <span class="comment">//checks if number is even</span></span><br><span class="line">        fmt.Println(num,<span class="string">"is even"</span>) </span><br><span class="line">    &#125;  <span class="keyword">else</span> &#123;</span><br><span class="line">        fmt.Println(num,<span class="string">"is odd"</span>)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在上面的程序中，<code>num</code> 在 <code>if</code> 语句中进行初始化，<code>num</code> 只能从 <code>if</code> 和 <code>else</code> 中访问。也就是说 <code>num</code> 的范围仅限于 <code>if</code> <code>else</code> 代码块。如果我们试图从其他外部的 <code>if</code> 或者 <code>else</code> 访问 <code>num</code>,编译器会不通过。</p>
<p>让我们再写一个使用 <code>else if</code> 的程序。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (  </span><br><span class="line">    <span class="string">"fmt"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;  </span><br><span class="line">    num := <span class="number">99</span></span><br><span class="line">    <span class="keyword">if</span> num &lt;= <span class="number">50</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"number is less than or equal to 50"</span>)</span><br><span class="line">    &#125; <span class="keyword">else</span> <span class="keyword">if</span> num &gt;= <span class="number">51</span> &amp;&amp; num &lt;= <span class="number">100</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"number is between 51 and 100"</span>)</span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"number is greater than 100"</span>)</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在上面的程序中，如果 <code>else if num &gt;= 51 &amp;&amp; num &lt;= 100</code> 为真，程序将输出 <code>number is between 51 and 100</code>。</p>
<h3 id="一个注意点"><a href="#一个注意点" class="headerlink" title="一个注意点"></a>一个注意点</h3><p><code>else</code> 语句应该在 <code>if</code> 语句的大括号 <code>}</code> 之后的同一行中。如果不是，编译器会不通过。</p>
<p>让我们通过以下程序来理解它。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (  </span><br><span class="line">    <span class="string">"fmt"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;  </span><br><span class="line">    num := <span class="number">10</span></span><br><span class="line">    <span class="keyword">if</span> num % <span class="number">2</span> == <span class="number">0</span> &#123; <span class="comment">//checks if number is even</span></span><br><span class="line">        fmt.Println(<span class="string">"the number is even"</span>) </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="keyword">else</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"the number is odd"</span>)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在上面的程序中，<code>else</code> 语句不是从 <code>if</code> 语句结束后的 <code>}</code> 同一行开始。而是从下一行开始。这是不允许的。如果运行这个程序，编译器会输出错误，</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">main.go:12:5: syntax error: unexpected else, expecting &#125;</span><br></pre></td></tr></table></figure>

<p>出错的原因是 Go 语言的分号是自动插入。</p>
<p>在 Go 语言规则中，它指定在 <code>}</code> 之后插入一个分号，如果这是该行的最终标记。因此，在if语句后面的 <code>}</code> 会自动插入一个分号。</p>
<p>实际上我们的程序变成了</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> num%<span class="number">2</span> == <span class="number">0</span> &#123;  </span><br><span class="line">      fmt.Println(<span class="string">"the number is even"</span>) </span><br><span class="line">&#125;;  <span class="comment">//semicolon inserted by Go</span></span><br><span class="line"><span class="keyword">else</span> &#123;  </span><br><span class="line">      fmt.Println(<span class="string">"the number is odd"</span>)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>分号插入之后。从上面代码片段可以看出第三行插入了分号。</p>
<p>由于 <code>if{…} else {…}</code> 是一个单独的语句，它的中间不应该出现分号。因此，需要将 <code>else</code> 语句放置在 <code>}</code> 之后处于同一行中。</p>
<p>我已经重写了程序，将 else 语句移动到 if 语句结束后 <code>}</code> 的后面，以防止分号的自动插入。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (  </span><br><span class="line">    <span class="string">"fmt"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;  </span><br><span class="line">    num := <span class="number">10</span></span><br><span class="line">    <span class="keyword">if</span> num%<span class="number">2</span> == <span class="number">0</span> &#123; <span class="comment">//checks if number is even</span></span><br><span class="line">        fmt.Println(<span class="string">"the number is even"</span>) </span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        fmt.Println(<span class="string">"the number is odd"</span>)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>现在编译器会很开心，我们也一样 ?。</p>
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